Embodiments of the invention disclosed herein relate to circuit interrupters or circuit breakers. More specifically, embodiments of the invention relate to lowering the force required of sensors, such as fault detectors, in trip apparatus for circuit interrupters or circuit breakers.
Circuit interrupters or circuit breakers use various trip devices to detect a fault and open a circuit to which they are connected. The trip devices include sensors and activate an operating mechanism of the breaker that moves a movable contact out of engagement with a fixed contact when the fault is detected. Some circuit breakers are also configured to trip other circuit breakers remotely.
One type of trip device used in circuit breakers is an electromagnetic trip device, which is generally used to open the breaker during a surge event. An example of an electromagnetic trip device is a solenoid serially connected to a line conductor of the breaker and arranged to activate the operating mechanism when current in the line conductor exceeds a predetermined level.
Another type of trip device used in circuit breakers is a thermal trip device, which is generally used to open the breaker during an overload event. An example of a thermal trip device is a thermal element, typically a bimetallic element (bimetal), serially connected to a line conductor of the breaker and arranged to activate the operating mechanism when current in the line conductor has exceeded a predetermined level for a predetermined amount of time. This type of bimetal trip device is known in the art as a directly-heated bimetal. Other bimetal trip devices may be thermally connected to a line conductor through a heating element that itself is serially connected to the line conductor. This type of bimetal/heater trip device is known in the art as an indirectly-heated bimetal.
Many circuit breakers employ both electromagnetic and thermal trip devices in a so-called thermal-magnetic trip unit. In a thermal-magnetic trip unit, the electromagnet or the thermal element or both may be required to provide or overcome a relatively high trip force. The amount of force required to trip the mechanism of some breakers can be as much as 4 Newtons (N), and larger breakers can have much higher trip forces. Additionally, some arrangements have a trip bar, which is what the trip device is arranged to move, directly attached to the mechanism. This couples the mechanism and trip device(s).
Some designs use a secondary latching system, such as is used in many interchangeable trip unit designs, which can reduce the force required by the trip device(s) to trip the mechanism. In an interchangeable trip unit configuration, the trip device contacts a trip bar that is part of a secondary latching system containing stored energy in the form of springs. The electromagnetic or thermal trip device, or both, can then release this secondary latching system, which then trips the mechanism. This configuration reduces coupling between the trip device and the mechanism, but does not eliminate the coupling and adds a significant amount of complication to the design. The second latching system also adds cost. Additionally, though the force required to release the latching system is reduced, the required force is still somewhat large. For example, in a breaker requiring about 4 N to trip the mechanism, the second latching system can still require a relatively large force of about 2.5 N.
There is thus a need for a trip apparatus that decouples the apparatus from the operating mechanism and reduces the amount of force required from the fault detector(s) to trip the mechanism.
Many circuit breakers also use auxiliary trip systems. Auxiliary trip systems can be used in several ways, but are typically used to trip a breaker more rapidly than a primary trip device of the breaker. For example, a typical primary electromagnetic trip device can have an intentional delay, such as one cycle, to give a downstream breaker an opportunity to trip and eliminate a fault danger to the upstream breaker. This intentional delay may be disadvantageous in higher current surge events, and thus an auxiliary trip device can be employed to trip the breaker more rapidly under such circumstances.
Prior art auxiliary trip systems include, for example, pressure powered auxiliary trip systems and magnetic trip systems. Several design constraints make auxiliary trip systems particularly difficult to design. Most auxiliary trip systems must harvest residual energy in the breaker to create mechanical energy to trip the breaker. For example, in pressure powered auxiliary trip systems, breaker exhaust gas pressure is used as an energy source, and in magnetic trip auxiliary systems, magnetic force generated by current flow is used. In both example types, the auxiliary trip system must harvest enough energy to trip the mechanism and convert the residual energy to a relatively high amount of mechanical force, which may be difficult to accomplish, particularly for pressure powered auxiliary trip systems.
There is thus a need for an auxiliary trip system that requires less energy for operation and that is easier to tune.